1. Field of the Invention
The present invention relates to oil and/or gas production. More specifically, the invention is a differential shifting tool and method for selectively actuating a downhole device.
2. Description of the Related Art
In hydrocarbon wells, fracturing (or “fracing”) is a technique used by well operators to create or extend fractures from the wellbore deeper into the surrounding formation, thus increasing the surface area for formation fluids to flow into the well. Fracing is typically accomplished by either injecting fluids into the formation at high pressure (hydraulic fracturing) or injecting fluids laced with round granular material (proppant fracturing) into the formation. This requires selective actuation of downhole devices, such as fracing valves, to control fluid flow from the tubing string to the formation.
For example, U.S. Published Application No. 2008/0302538 (the '538 Publication), entitled Cemented Open Hole Selective Fracing System and which is incorporated by reference herein, describes one system for selectively actuating a fracing sleeve that incorporates a shifting tool. The tool is run into the tubing string and engages with a profile within the interior of the valve. An inner sleeve may then be moved to an open position to allow fracing or to a closed position to prevent fluid flow to or from the formation.
After the fracing process is complete and prior to the initiation of production operations, the ball and seat are typically milled out from each of the tools to allow a large flowpath through the producing string. After the milling process is complete, and as described in the '538 Publication, the shifting tool is disposed through the string and is caused to engage a profile within the downhole device, thus allowing the well operator to engage the moveable portion of the tool and close off the flow ports from the surrounding formation.
A common problem with conventional downhole devices during fracing and the milling process is the profile becomes damaged and/or destroyed. For example, it is not uncommon that the fracing process itself, which by its nature incorporates abrasive materials moving at high flow rates, erodes the engageable profile of the tool. To avoid this problem, well operators often limit the fracing flow rate to control erosion of the profile, which decreases the effectiveness of the fracing process and results in less than optimal results.